Several types of hydraulic cylinders of this type are known which are provided with contact-free electric position indicators for a remote display of the end positions of the piston. In order to obtain the contact-free establishment of contact magnetically operated switches are often mounted on the outside of the cylinder, the piston then necessarily comprising permanently magnetic material in order to actuate the switches. The latter implies, however, that the cylinder is made of non-ferromagnetic material encumbered with the draw-back that it cannot withstand particularly high operational pressures when the cylinder is to be manufactured at a reasonable price. Furthermore these switches are expensive and fails often in operation. The system is therefore only used for hydraulic cylinders operating at a substantially lower operational pressure than the usual 200-300 bar.
U.S. Pat. No. 3,453,937 discloses a hydraulic cylinder of this type, and whereby a bore is provided in the housing of the cylinder for the mounting of an inductive sensor. The bore is closed towards the interior of the cylinder by a remaining part of the wall of the cylinder. As a result the housing of the cylinder must be made of a non-magnetizable material, preferably aluminium, so that the inductive sensor is able to transmit a magnetic field through the remaining portion of the wall of the cylinder. The use of aluminium implies that the housing of the cylinder must be manufactured with great dimensions in order to withstand the usually high operational pressures of the hydraulic cylinder. This hydraulic cylinder is therefore very expensive to manufacture and furthermore it is impossible to measure through the required thickness of the remaining part of the wall of the cylinder by means of a small inductive sensor.
Inductive sensors to be built into pressure containers and capable of withstanding pressures up to about 600 bar are, however, known. These sensors are, however, very expensive compared to usual electric position indicators, and furthermore they are of such a size--of a diameter of between 12 and 16 mm--that they can only be mounted in very great hydraulic cylinders. Such an inductive sensor controls the state of the linings, pistons, and piston rings in combustion engines and is known from Danish printed accepted specification No. 146,422. This inductive sensor is mounted in a container with a bottom. It is, however, necessary that the bottom is of a thickness of between 5 and 7 mm, and such a thickness never allows a measuring by means of a small inductive sensor.
In the frequently used hydraulic cylinders of a diameter of about 25 to 125 mm it is possible to mount inductive sensors of a diameter of about 4 mm. These small inductive sensors can, however, only withstand an operational pressure of up to about 70 bar, which does not suffize because the use of the normal operational pressure of 200-300 bar necessitates that the sensor can withstand pressure limits of up to 600-800 bar. Consequently, it is a great drawback that previously it has been impossible to provide hydraulic cylinders, especially of diameters of between 25 and 125 mm, and comprising electric position indicators at a reasonable price, and which can withstand the normal operational pressures.